1. Field of the Invention
The present invention relates to a brightness controller for neon tubes, fluorescent lamps or other discharge tubes. In particular, the invention relates to a discharge tube brightness controller which can continuously control the brightness of the discharge tubes and drive the discharge tubes with low power dissipation.
2. Description of the Related Art
Brightness control is to change the light flux from a light source by adjusting the input to the source. It is possible to control the brightness of an incandescent lamp continuously between 100% (rated light flux) and 0% by varying the input voltage. Recently, in many cases, the incandescent lamp has been driven with pulses by a thyrister connected in series to the lamp. The ON time of the pulses is controlled to change the average current, controlling the brightness of the lamp. On the other hand, a discharge tube such as a neon tube and a fluorescent lamp has a narrow voltage range for stable discharge. It is therefore impossible to control the brightness of a discharge tube by controlling the input voltage. Normally, the input current is changed to control the discharge tube brightness. In this case, it is necessary to keep thermoelectrons emitted from the discharge tube electrodes heated specially. In general, a voltage which is higher than a certain value is input as pulses. The ON time of the pulses is changed to control the average current. This brightness control is performed by changing the ratio of the ON time to the OFF time at a constant frequency.
As well known, the impedance of the discharge tube which has not started to discharge varies greatly from that of the tube which has started to. As also known, the impedance of the discharge tube depends on the length and diameter of the tube, the input voltage and external factors. Therefore, the conditions for driving the discharge tubes are different for different tubes, and no general purpose continuous brightness controller is known. Even in a case where the input voltage of the discharge tube is driven with pulses, the range of stable discharge is limited. Therefore, practically, it is impossible to control the brightness of the discharge tube continuously over a wide dynamic range. In particular, it is difficult to control the brightness of a high voltage discharge lamp. Generally, the brightness of such a lamp can be controlled only in two, three or more steps. On detailed study of the transient response of the discharge tube being driven with pulses, it is found that the input voltage applied to the tube does not contribute all the time to discharge. In some cases, considerable components of such voltage is wasted as heat.
In particular, of the discharge tubes, neon tubes vary in length and diameter, and differ in impedance. In some cases, it was not possible to light or turn on all of the discharge tubes different in impedance which are connected to a particular lighting device, because the impedances and/or other discharging or luminous conditions vary tube by tube and a limited condition of a single lighting device cannot match these conditions. Each such discharge tube was turned on under conditions not matching with the impedance of the circuit including the tube, a high frequency transformer and an oscillating capacitor, and with the frequency of energy variation inherent in the circuit. Therefore, even if it was possible to turn on the discharge tubes, the efficiencies were generally low.
Because conventional brightness control is performed at a constant frequency, the ON time and the OFF time are related to each other, and the range of brightness control is narrow. In this case, because both the ON time and the OFF time change, it is impossible to keep the ON time constant. Consequently, the ON time becomes more and more deviate from the optimum value, lowering the efficiency of brightness control.